1. Barely enough time to complete levels, which is annoying.
2. Time is a dumb concept in games at all, there is a reason it is no longer used.
3. Controls are not very good jumping in off due to such sudden drops, and you can jump into ceiling and get stuck until you come back down, chipping away at your already limited time.
4. Bugs. I ran into several invisible walls, and failed a mission despite getting to the portal on time.
5 HUD got in the way alot, especially when you get all the buttons and the banner cuts out the entire center of the game for three seconds, and of course the time is still going down then even though you can't see.
These are some serious problems, and I suggest you fix them, though whole concept has been done to death already anyway.